The present invention relates to a synchronization detection system and, more particularly, to one for use in data transmission in which two-dimensionally modulated signals are received by way of a demodulator.
As well known in the art, where facsimile signals, data and like digital signals are to be transmitted over an analog transmission line such as a telephone line, they are often subjected to quadrature amplitude modulation (QAM) which uses a modem. At the time of receipt of a signal from a transmission line, receipt of an actual information signal is preceded by a training sequence for the modem in order to set up synchronization, that is, initializing the modem to converge the parameters associated with an equalizer, automatic gain control and other various functions.
For example, in accordance with the CCITT (Consultive Committee of International Telegraph and Telephone) Recommendation V. 29, an alternating repetitive pattern of binary symbols, i.e. alternation, is transmitted at the beginning of a training sequence (segment 2), followed by an equalizer setting pattern for converging various parameters of an equalizer (segment 3). Concerning the alternation prescribed by the Recommendation V. 29, as shown in a signal space with an in-phase axis I and a quadrature axis Q in FIG. 1, the initial symbol element A has a relative amplitude of 3 and a phase of 180 degrees with respect to the reference phase, while the second symbol element B has a relative amplitude of 3.sqroot.2 and a phase of 315 degrees at, for example, a data rate of 9,600 bits/second.
The procedure advances to the segment 3 after the above-stated alternation has occurred over a period of 128 symbols. In the segment 3, as shown in FIG. 2, one element C has a relative amplitude of 3 and a phase of 0 degree with respect to the reference phase, while the other element D has a relative amplitude of 3.sqroot.2 and a phase of 135 degrees at, for example, a data rate of 9,600 bits/second. The segment 3 comprises a pseudorandom sequence of such elements C and D.
The transition from the alternation to the pseudorandom sequence may be detected by discriminating the binary symbol of AB or CD at the modem of a receiving station. However, in order to effect fast convergence at the time of initializing the tap gain of an equalizer, a modem at a receiving station generally generates reference symbols and compares them with an equalizer initializing pattern to adjust the tap gain, instead of discriminating the binary symbols as described.
In detail, in the event of correcting the tap coefficient of an equalizer, the intersymbol interference is considerable and the adjustment of timing or that of carrier phase is insufficient at an initial stage of the training, rendering the discrimination of a binary symbol sometimes inaccurate. This is the reason for employing the method in which a receiving station generates as a reference signal a symbol sequence identical with a pseudorandom sequence which a transmitting station has sent, thereby correcting the tap coefficient of an equalizer. This method is practicable because the training sequence is known by the receiving station. The pseudorandom sequence generated by the receiving station has to be synchronous with the received symbol sequence and, for such synchronization, a transition point in the training sequence has to be accurately detected.
Japanese Patent Laid-Open Publication No. 52-89407/1977, for example, proposes a method which produces a vector sum of two samples which are continuous in a signal space, and detects a time point when the real number portion of the vector sum becomes maximum. In another prior art method, an envelope signal is produced by the square sum of a demodulated in-phase signal and quadrature signal and the transition point is detected in terms of a rise of the envelope signal beyond a certain level (e.g. Japanese Patent Laid-Open Publication No. 56-1631/1981). In still another prior art method, a received signal is multiplied by a carrier component so that the transition point may be identified in terms of inversion of the polarity of the product (e.g. Japanese Patent Laid-Open Publication No. 56-119562/1981).
The problem encountered with any of the above-discussed prior art methods is that, where the distortion on the transmission line is substantial, symbols before and after a point of signal change greatly interfere with each other and, particularly at the initial stage of training, the intersymbol interference and timing error enhance the signal distortion. The result is an error on the order of .+-.1 symbol in terms of time in the detection of a transition point from the segment 2 to the segment 3. In the case of a system which picks up timing information necessary for controlling a modem from the taps of an equalizer, the error on the order of .+-.1 symbol in the detection of a transition point makes it impossible to pick up correct timing information and often causes deviation of synchronization by a period of one to two samples, disenabling the modem to function properly.